Comment # 3 on bug 1205740 from William Brown

No, this is not expected. Btrfs here is fundamentally confusing a "filesystem"
(the mountpoint and the data within it) from the volume group (the collection
of disks that provide space to the set of contained filesystems).

This operation to re-balance is operating on the *volume group* and is not
changing properties of the filesystem. Since "/" is listed as the default
administration point of the volume group here, this means that on a
transactional server, operations to either administer, repair or interact with
the volume group will fail *incorrectly*.

Also /home and / are the same volume group, but not the same filessytem. So the
fact that a command which operates on the volume group fails on one filessytem,
but not the other is inconsistent and confusing.